This application claims the benefit of priority under 35 U.S.C. xc2xa7119 of Russian Federation Patent application Serial No. 2001111056.
This application is related to co-filed application, Ser. No. 10/124,546, which is hereby incorporated by reference.
The present invention relates generally to optical fluoride crystals for transmitting below 200 nm UV region light, and particularly to UV region  less than 200 nm transmitting calcium fluoride crystals for excimer laser lithography optics.
The present invention belongs to the technical field of preparing artificial crystals of optical-grade fluorite by growing its monocrystals from a melt, cooling them in a temperature gradient, and using a crystal nucleus for the growth.
Optical-grade fluorite crystals are grown industrially by Stockbarger""s method, in which a crucible containing the melt is basically moved through a temperature domain with a gradient, under a high vacuum.
Two isothermal regions, with a temperature drop in between, are generally used when growing crystals in a downward moving crucible. This makes it possible to anneal the crystals straight after growing them, without subjecting them to the high thermal stresses that occur when there is an extremely great temperature drop. The furnace used here should have two temperature zones, with a minimal heat exchange between them. For this purpose, these zones are separated by a thermal insulator and a shielding screen, and the temperature in the two zones can be regulated independently from each other (see R. Lodiz* and R. Parker: xe2x80x9cRost kristallovxe2x80x9d [Growth of Crystals], translated from English into Russian, edited by A. A. Chemov and published by Mir in Moscow in 1974, p. 181).
A process for growing calcium fluoride crystals has been described in the book entitled xe2x80x9cOpticheskiy flyuoritxe2x80x9d [Optical-Grade Fluorite] by N. P. Yushkin et al., published by Nauka in Moscow in 1983 (see pp. 83 and 84). This process includes the prior preparation, in which the apparatus and the crucible are first cleaned with compressed air, the crucible is filled with fluorite fragments, and the apparatus containing the crucible is put under a vacuum of 1xc3x9710xe2x88x924 mm Hg. The crucible is then heated to 1500xc2x0 C. at a rate of 5xc2x0 C./minute in the course of 4-5 hours. The material is kept at this holding temperature until it is completely melted and the melt is fully homogeneous, the time of which depends on the size of the crucible and can be up to 20 hours. The crucible containing the melt is then automatically lowered at a speed of 2-20 mm/hour.
The crystals are grown here in the crystallization zone at a constant crystallization temperature of 1450xc2x0 C. for 10-15 hours, and the temperature of the furnace is then reduced to room temperature according tot a fixed temperature program. The total crystal-growing time is 30-50 hours. A single-stage annealing, carried out in the same crystal-growing apparatus, is recommended for optimal results. For the annealing of the crystals, the temperature in the upper zone of the furnace is reduced to a value of between 80 and 1150xc2x0 C., depending on the size of the crystals. The crucible containing the crystals is then raised again to its initial position in the upper zone and kept there for 5-10 hours. The temperature is then reduced to 250-150xc2x0 C. at a rate of 3-25xc2x0 C./hour. The heating is finally stopped, and the crystals are allowed to cool down naturally over the rest of the temperature range.
However, the disadvantage of this process is that the crystal preparation is discontinuous, and the crystals have to be heated twice in the upper zone. This can lead to stresses in the resulting monocrystals and to the formation of blocks in them.
The process that is known from the prior art and which can be regarded as the one that is the most like the process according to the present invention is the crystal-growing method for calcium fluoride described in the first reference mentioned above, i.e. the book by R. Lodiz* and R. Parker: xe2x80x9cRost kristallovxe2x80x9d [Growth of Crystals], translated from English into Russian, edited by A. A. Chemov and published by Mir in Moscow in 1974 (see p. 181 and p. 188 there).
In the process described in this book, the melt is cooled at a rate of at least 7xc2x0 C./cm, and the crucible is made to descend at a speed of 1-5 mm/hour.
However, the details for the concrete realization of the crystal-growing procedure are not given in the description of any of the above processes.
Yet the process parameters must be carefully and specially chosen for all stages of the crystallization and the annealing if one is to obtain high-quality fluorite crystals that are suitable for the preparation of optical components with the required characteristics.
An aspect of the present invention is to prepare calcium fluoride monocrystals with a high optical uniformity (having a xcex94n value of 1xc3x9710xe2x88x926) and a low birefringence (having a xcex4 value of 1-3 nm/cm).
An aspect of the invention is achieved by optimizing the conditions of the technological process involved in the entire cycle of growing monocrystals. The process involves crystallization from the melt and the annealing and cooling of the crystals, in a vacuum furnace. The crucible containing the melt is continuously moved from the crystallization zone into the annealing zone, and each of the two zones is fitted with its own independent means for controlling the process parameters. Unlike in the process according to the prior art, there is here a 250-450xc2x0 C. temperature drop from the crystallization zone to the annealing zone, with a gradient of 8-12xc2x0 C./cm. The crucible containing the material to be crystallized is moved from the crystallization zone to the annealing zone at a speed of 1-3 mm/hr. The material is first kept in the annealing zone at a holding temperature of 1100-1300xc2x0 C. for 20-40 hours, and then it is cooled first to 950-900xc2x0 C. at a rate of 2-4xc2x0 C./hour, and then further to 300xc2x0 C. at a rate of 5-8xc2x0 C./hour, the rest of the cooling being allowed to occur naturally.